CPR chest compression monitor

ABSTRACT

Chest compressions are measured and prompted to facilitate the effective administration of CPR. A displacement detector produces a displacement indicative signal indicative of the displacement of the CPR recipient&#39;s chest toward the recipient&#39;s spine. A signaling mechanism provides chest compression indication signals directing a chest compression force being applied to the chest and a frequency of such compressions. An automated controller and an automated constricting device may be provided for applying CPR to the recipient in an automated fashion. The automated controller receives the chest compression indication signals from the signaling mechanism, and, in accordance with the chest compression indication signals, controls the force and frequency of constrictions. The system may be provided with a tilt compensator comprising a tilt sensor mechanism outputting a tilt compensation signal indicative of the extent of tilt of the device, and may be further provided with an adjuster for adjusting the distance value in accordance with the tilt compensation signal. An ECG signal processor may be provided which removes the CPR-induced artifact from a measured ECG signal obtained during the administration of CPR.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device for aiding in theadministration of cardiopulmonary resuscitation (CPR). Morespecifically, certain aspects of the invention relate to devices formonitoring CPR efforts and facilitating better CPR administration.

2. Description of Related Art

Various U.S. patent documents disclose sensors for assisting in theadministration of CPR. For example, U.S. Pat. No. 5,589,639 (D'Antonioet al.) discloses a force sensing system for a CPR device whichgenerates an intelligible output signal corresponding to a forceparameter. The CPR device utilizes a signal indicative of the forcebeing applied to the recipient's chest.

U.S. Pat. No. 5,496,257 (Kelly) discloses an apparatus for assisting inthe application of CPR. The device rests on the recipient's chest. Chestcompression forces are monitored by the device in order to ascertain therate of compression and blood flow. This information is activelyprovided to the rescuer to prompt proper administration of CPR.

Various devices are disclosed which assist in the timing of theapplication of CPR, including U.S. Pat. No. 5,626,618 (Ward et al.) andU.S. Pat. No. 4,863,385 (Pierce). The '618 patent discloses, among otherthings, an electrode combination for cardiac pacing and cardiacmonitoring in association with a bladder for use in the patient'sesophagus for improving artificial circulation as a result of CPR. The'385 patent discloses a CPR sequencer which comprises a compact,portable, computer-controlled device, which provides timing and sequenceguidance for helping a rescuer in the application of CPR to a recipient.

Each year there are more than 300,000 victims of cardiac arrest. Currentconventional techniques for CPR introduced in 1960 have had limitedsuccess both inside and outside of the hospital, with only about 15%survival rate. Accordingly, the importance of improving resuscitationtechniques cannot be overestimated. In the majority of cardiac arrests,the arrest is due to ventricular fibrillation, which causes the heart toimmediately stop pumping blood. To treat ventricular fibrillation,defibrillation is administered which involves the delivery of a highenergy electric shock to the thorax to depolarize the myocardium, and toallow a perusing rhythm to restart. If, however, more than a few minutespass between the onset of ventricular fibrillation and the delivery ofthe first defibrillation shock, the heart may be so deprived ofmetabolic substrates that defibrillation is unsuccessful.

The role of CPR is to restore the flow of oxygenated blood to the heart,which may allow defibrillation to occur. A further role of CPR is torestore the flow of oxygenated blood to the brain, which may preventbrain damage until the heart can be restarted. Thus, CPR is critical inthe treatment of a large number of patients who fail initialdefibrillation, or who are not candidates for defibrillation.

Various studies show a strong correlation between restarting the heartand higher levels of coronary blood flow. To restart the heart, ifinitial defibrillation fails (or is not indicated), coronary flow mustbe provided. With well-performed CPR, together with the use ofepinephrine, brain blood flow probably reaches 30-50% of normal.Myocardial blood flow is much more limited, however, in the range of5-20% of normal. In patients, heart restarting has been shown tocorrelate with the pressure gradient between the aorta and the rightatrium, obtained between compressions (i.e., the coronary perfusionpressure). CPR, when applied correctly, is designed to provide asufficient amount of coronary perfusion pressure by applying asufficient amount of chest compression force. Unfortunately, however,studies indicate that CPR is performed correctly only part of thetime—approximately 50% of the time according to a study conducted on 885patients. Hoeyweghen et al., “Quality and Efficacy of Bystander CPR,”Resuscitation 26 (1993), pp. 47-52. The same study showed that long-termsurvival, defined as being awake 14 days after CPR, was 16% in patientswith correct CPR, but only 4% when CPR was performed with less chestcompression (p<0.05). Thus, properly administered CPR can increasesurvival rates.

Not only is the correct application of CPR critical to the survival ofthe CPR recipient, but when initial defibrillation is unsuccessful, oris not indicated, it can be essential that CPR be applied immediately.The sooner persons are resuscitated, the more likely they will survivelong-term with preservation of neurologic function. When initialresuscitative efforts at the scene of an arrest fail to restore nativecardiac function, it is often the practice to transport the patient tothe hospital with the hope that better CPR can be performed under thesupervision of a physician. A number of studies have shown, however,that it is quite rare for a patient who is not resuscitated in the fieldto be resuscitated in the hospital, and survive with meaningfulneurologic function. Even invasive interventions used in hospitals, suchas open chest cardiac massage, have failed to improve survival rates,probably due to irreversible organ damage produced by prolonged schemaduring transportation.

The American Heart Association (AHA) published guidelines specify thatchest compression during CPR should be done at a rate of 80-100compressions per minute at a depth of 1.5 to 2 inches. During CPRcourses, instrumented mannequins are generally used that measure theamount of chest compression a student applies. It is then up to thestudent to apply similar chest compressions in an emergency situation,without feedback, relying only on the feel and look of the compressions.Since there is no feedback, and since relatively small changes in theamount of compression can affect perfusion pressure, it is notsurprising that CPR is often performed incorrectly.

As described above, various types of devices have been provided to helpgive the rescuer administering CPR feedback. However, these devices donot measure chest displacement. Rather, they measure compression forceas a result of the applied CPR. This is problematic since with clinicalCPR there is considerable variation in the compliance of differentpatients'chests, such that similar compression forces producesubstantially different chest displacements in different patients.

Gruben et al. disclose in their article entitled “SternalForce-Displacement Relationship During Cardiopulmonary Resuscitation,”Journal of Biomedical Engineering, Volume 115 (May 1993), p. 195, theuse of mechanical linkages incorporating position-sensing transducers tomeasure chest displacement during clinical CPR. However, this mechanismpresents problems in general clinical environments, such as delays insetup and awkward handling.

While resuscitation is in progress, it is vital that physicians,paramedics, and other healthcare professionals administering CPR becontinuously aware of changes in the patient's electrocardiogram (ECG),particularly the heart rhythm. An incorrect assessment of the heartrhythm can lead to administration of inappropriate therapy orwithholding of appropriate therapy. The chest compressions associatedwith CPR, however, introduce artifacts in the measured ECG signal thatmake its interpretation difficult. The rather inadequate approachgenerally used to facilitate ECG interpretation during CPR isintermittent cessation of chest compressions to provide a period ofartifact-free ECG acquisition. Problems occur with this approach. Forone, there is a loss of hemodynamic support when chest compressions arestopped. In addition, the ECG remains difficult or impossible tointerpret once chest compressions are resumed. Accordingly, suddenchanges in rhythm may not be appreciated until after a substantialdelay. In addition, since survival from cardiac arrest has been shown tobe related to blood flow generated during CPR, and since interruption ofchest compressions will reduce blood flow, survival may very well becompromised by these interruptions.

The outcome of CPR may be improved if there were a means for reducingthe CPR-induced artifacts present in an ECG signal in a manner whichwould allow the correct interpretation of the ECG without interruptingchest compressions applied during CPR. E. Witherow has performed studieswhich demonstrate that CPR-induced artifacts are due primarily tochanges in the half-cell potential of electrodes, caused by theirmechanical disturbance. This was published in a thesis entitled A Studyof the Noise in the ECG During CPR, M.S. thesis, the Johns HopkinsUniversity (1993), the content of which is hereby expressly incorporatedby reference herein in its entirety.

There is a need for compact, portable, and economic tools for monitoringCPR efforts, aiding in the correct administration of CPR, and otherwiseincreasing the success of resuscitation efforts, e.g., by removingCPR-induced artifacts from ECG signals so CPR does not need to bestopped in order to obtain a good ECG reading.

SUMMARY OF THE INVENTION

The present invention is provided to aid in the proper application ofCPR in various situations in order to substantially improve the survivalrate of CPR recipients. The present invention is also provided toimprove upon resuscitation techniques involving the concurrentadministering of CPR and monitoring of the patient's ECG, and moreparticularly, the patient's heart rhythm. In order to achieve this end,one or more aspects of the present invention may be followed in order tobring about one or more specific objects and advantages, such as thosenoted below.

One object of the present invention is to provide a system or device formeasuring and prompting chest compressions to facilitate the effectiveadministration of CPR.

A further object is to provide a hand-held CPR chest compression monitorwhich accurately measures the rate and depth of chest compressionsduring the administration of CPR. In accordance with one aspect of theinvention, the device signals the rescuer to prompt correctcompressions. An object of the present invention is to provide such adevice which only requires a minimum amount of set-up time, is intuitivein its operation, and is easy to use. The device would preferably besmall in size, have a low weight, and be inexpensive to manufacture anddistribute.

In accordance with another aspect of the present invention, a hand-heldCPR chest compression monitor is provided with an integral defibrillatorand/or data storage and retrieval components. Another object of thepresent invention is to provide a system for concurrently administeringCPR with the aid of a hand-held CPR chest compression monitor andobtaining ECG signals from the CPR recipient, and to provide a devicefor removing compression-induced artifacts found in the ECG signalsduring CPR to allow accurate ECG and heart rhythm readings withoutstopping CPR.

The present invention, in accordance with one aspect, is thereforedirected to a system (which may be in the form of a hand-held device),for measuring and prompting chest compressions to facilitate theeffective administration of CPR by a rescuer. The system comprises adisplacement detector for producing an output signal indicative of adisplacement of a CPR recipient's chest toward the CPR recipient'sspine. A signaling mechanism is provided for providing signals directinga chest compression force being applied to the chest and a frequency ofcompressions to bring and maintain the frequency of compressions withina desired frequency range and to bring and maintain the chestdisplacement within a desired distance range.

In accordance with an aspect of the invention, the displacement detectorcomprises a motion detector for determining an amount of motion of thechest in relation to the spine. A converter may be provided forconverting an output signal produced by the motion detector into adistance value. The signaling mechanism may comprise a mechanism forcomparing the distance value to a desired range of distance values, andfor signaling directions regarding chest compression force and frequencyin accordance with whether the value falls within the desired range ofdistance values.

A hand-held CPR chest compression monitor such as that provided abovemay be used in association with an automated chest compression mechanismto control the manner in which the automated chest compression mechanismapplies chest compressions to a recipient to thereby effectivelyadminister CPR to the recipient in accordance with certain chestdisplacement and compression frequency parameters. Such a hand-held CPRchest compression monitor may be further provided in association with anECG monitor. An ECG signal enhancer may be provided for subtracting orotherwise suppressing chest compression-induced artifacts from the ECGsignal, to facilitate reading of the ECG signal, and more particularly,to facilitate reading of the heart rhythm of the CPR recipient withoutthe need to stop CPR.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, and advantages of the presentinvention are further described in the detailed description whichfollows, with reference to the drawings by way of non-limiting exemplaryembodiments of the present invention, wherein like reference numeralsrepresent similar parts of the present invention throughout the severalviews and wherein:

FIG. 1 is a schematic diagram of an exemplary embodiment of a hand-heldCPR chest compression monitor;

FIG. 2 is a waveform diagram comparing measured and calculated signalscaused by manual compressions of a simulated chest of a CPR recipient;

FIG. 3 is a perspective view of an exemplary mechanical layout of ahand-held module for monitoring CPR chest compressions;

FIG. 4 is a side view of the module illustrated in FIG. 3;

FIGS. 5-7 each show a rescuer administering CPR to a CPR recipientutilizing various embodiments of a CPR monitoring device according thepresent invention;

FIG. 8 shows a CPR recipient coupled to various resuscitation assistanceapparatuses;

FIG. 9 is a flow chart of the process utilized by the chest compressionmonitor illustrated in FIG. 1 in order to convert a detectedacceleration signal into a displacement value;

FIG. 10. is a schematic diagram illustrating a model of a systemcomprising a CPR recipient receiving CPR while an ECG monitor connectedto the CPR recipient generates a measured ECG signal e_(m);

FIG. 11 provides a model for the conversion of a measured ECG signal emto a processed measured ECG signal e_(m)′;

FIG. 12 is a waveform diagram showing the respective waveforms a_(r),e_(m), a_(p), e_(m)′;

FIG. 13 shows another model of a system involving the administration ofCPR to a patient, monitoring of chest compressions, producing a measuredacceleration signal, and producing a measured ECG signal;

FIG. 14 illustrates a mechanism provided in accordance with a specificaspect of the invention for identifying a system and using theidentified system to produce a processed measured ECG signal which isprocessed to remove a CPR-induced artifact;

FIG. 15 shows a block diagram representation of a first embodiment ECGsignal processor; and

FIG. 16 shows a block diagram representation of a second embodiment ECGsignal processor.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS

FIG. 1 is a schematic representation of one embodiment of a hand-heldCPR chest compression monitor 10 for measuring the rate and depth ofchest compressions during the administration of CPR. The illustratedmonitor 10 is a specific implementation of a monitoring system formeasuring and prompting chest compressions to facilitate the effectiveadministration of cardiopulmonary resuscitation (CPR). The systemcomprises a displacement detector and a signaling mechanism. Thedisplacement detector produces and outputs a displacement-indicativesignal indicative of the displacement of a CPR recipient's chest towardthe recipient's spine. The signaling mechanism provides chestcompression indication signals directing a chest compression forceapplied to the chest and a frequency of compressions to bring andmaintain the frequency and chest displacement parameters within desiredranges. The monitoring system may be further provided with a tiltcompensator comprising a tilt sensor mechanism outputting a tiltcompensation signal indicative of the extent of tilt of the device. Thesystem may further include an adjuster for adjusting the displacementvalue calculated from the measured acceleration signal in accordancewith the output tilt compensation signal.

In the illustrated implementation, a hand-held CPR chest compressionmonitor 10 is provided. It comprises a displacement detector comprisingan accelerometer 12 coupled to a microprocessor 28 via an interface 26.The illustrated interface 26 may comprise a parallel or serial interfacewhich may be internal (where microprocessor 28 is provided as part ofone integral device) or external (where microprocessor 28 is provided asa separate device). The signaling mechanism comprises an audibleindicator (i.e., a loud speaker) 18, which has an input connected tomicroprocessor 28 via interface 26. A DC voltage power supply 20 isconnected between a switch 22 and ground, and provides a DC voltage +Vfor powering the various components of the illustrated monitor 10,including the above-noted accelerometer 12 and audible indicator 18.Tilt compensation devices are provided which include a first gyro 24 anda second gyro 25. They each include outputs connected to microprocessor28 via interface 26.

While the illustrated monitor uses an audible indicator, other types ofindicators may be used in addition or as an alternative. For example,the indicator may comprise a vibrating mechanism, visual indicators(e.g., blinking LEDs), and so on.

The illustrated monitor 10 determines chest displacement from a doubleintegration of an acceleration signal produced by accelerometer 12.Microprocessor 28 is provided to handle the calculations needed toperform the various functions of the illustrated monitor 10, includingthe double integration of the acceleration signal. The accelerometer 12will preferably comprise a high-quality, inexpensive accelerometer, suchas the Analog Devices ADXL05.

The ADXL05 accelerometer comprises a complete acceleration measurementsystem provided on a single monolithic IC. It comprises a polysiliconsurface micro-machined sensor and signal conditioning circuitry whichimplement a force-balanced control loop. The accelerometer is capable ofmeasuring both positive and negative acceleration to a maximum level ofplus or minus 5 g. The sensor comprises 46 unit cells and a common beam.The unit cells make up a differential capacitor, which comprisesindependent fixed plates and central plates attached to the main beamthat moves in response to an applied acceleration. These plates form twocapacitors, connected in series. The sensor's fixed capacitor plates aredriven differentially by two 1 MHz square waves: the two square waveamplitudes are equal but are 180 degrees out of phase from one another.When at rest, the values of the two capacitors are the same, andtherefore, the voltage output at their electrical center (i.e., at thecenter plate) is 0. When there is an applied acceleration, the commoncentral plate or “beam” moves closer to one of the fixed plates whilemoving farther from the other. This creates a miss-match in the twocapacitances, resulting in an output signal at the central plate. Theamplitude of the output signal varies directly with the amount ofacceleration experienced by the sensor.

A self-test may be initiated with the ADXL05 accelerometer by applying aTTL “high” level voltage (>+2.0 Vdc) to the accelerometer self-test pin,which causes the chip to apply a deflection voltage to the beam whichmoves it an amount equal to −5 g (the negative full-scale output of thedevice).

In operation, accelerometer 12 of compression monitor 10 will move invarious directions not limited to a simple vertical-only movement. Inother words, monitor 10 will tilt on the CPR recipient's chest duringthe administration of CPR, which will cause the linear motion indicatedby accelerometer 12 to be corrupted by non-linear tilt-inducedmovements. Accordingly, the above-described tilt sensor mechanism 16 isprovided in the illustrated embodiment, to facilitate the determinationof the true displacement of the chest in relation to the recipient'sspine without errors caused by tilting of the device with respect to thechest. First gyro 24 produces an angular velocity signal indicating themeasured angular velocity around a first horizontal longitudinal axis,and second gyro 25 outputs an angular velocity signal indicating themeasured angular velocity around a second horizontal longitudinal axispositioned perpendicular to the first longitudinal axis. These angularvelocity signals integrated to obtain angular displacement signals,which can be used to correct the measured linear displacement for tiltof the monitor 10.

First and second gyros 24 and 25 may comprise a Murata Gyrostar(piezoelectric gyroscope (ENC05E). This commercially available gyro isapproximately 20×8×5mm in size, and is designed for large-volumeapplications such as stabilizing camcorder images. This gyro uses theCoriolis principle, which means that a linear motion with a rotationalframework will have some force that is perpendicular to that linearmotion. The Coriolis force is detected and converted to a voltage outputby piezoelectric transducer elements mounted on a prism bar. The voltageoutput is proportional to the detected angular velocity. In theillustrated embodiment, the two gyros are driven at slightly differentfrequencies in order to avoid interference.

Interface 26, in addition to a serial or parallel interface, may furthercomprise A/D and D/A converters, including a D/A converter for drivingaudio transducer 18 to indicate the amount of displacement and to promptCPR at the correct rate (80-100 compressions per minute). The outputfrom accelerometer 12 is routed through an A/D converter provided aspart of interface 26 for digitization and subsequent analysis bymicroprocessor 28. Similarly, the output from each of first and secondgyros 24 and 25 is routed to microprocessor 28 via an A/D converterprovided as part of interface 26.

Microprocessor 28 is provided as part of a hand-held integrated modulecomprising monitor 10. As an alternative, a separate computer such as alap top computer may be provided which is coupled to interface 26(serving as an external interface) of the illustrated monitor 10.

Further information, regarding other types of inertial proprioceptivedevices utilizing accelerometers and gyros, is provided by C. Verplaetsein an article entitled “Intertial Proprioceptive Devices:Self-Motion-Sensing-Toys and Tools”, IBM Systems Journal, Vol. 35, Nos.3 and 4 (1996) pages 639-650, the content of which is hereby expresslyincorporated herein by reference in its entirety.

FIG. 2 shows signals produced by a simulated recipient chest assembly.The simulated chest assembly was comprised of a spring connecting ablock to a firmly supported base. Linear bearings where provided insidethe block rode on a shaft to keep the block aligned vertically and tofacilitate vertical movement of the block. A damper was coupled to theblock to slow the movement of the block to simulate chest compliance.Vertical displacement of the block was measured by a position transducer(LVDT). A force transducer was attached to the top of the aluminumblock, and provided signals indicative of the output forces as a resultof CPR-like compressions. The assembly was calibrated and designed toclosely mimic the visco-elastic properties of the human chest. The forcetransducer was calibrated with standard weights, and the displacementtransducer was calibrated with a ruler. An accelerometer (ANALOGDEVICES® ADXL-05) was mounted on a circuit board with appropriatebiasing and filtering components, and the circuit board was attached toan aluminum holder. The accelerometer assembly was placed on thesimulated chest and manual compressions were applied.

FIG. 2 shows a comparison of actual displacement (measured by LVDT) anddisplacement calculated using the acceleration signals from theaccelerometer assembly, during manual compressions of the simulatedchest. The acceleration signals were doubly integrated and were plottedwith the measured displacement and acceleration. The illustrated signalswaveforms are displayed with respect to an abscissa representing aprogression in time and an ordinate axis representing a value of eitherdisplacement in millimeters or acceleration g. The illustrated waveformsinclude an acceleration signal 30, a measured distance signal 32, and acalculated distance signal 34. FIG. 2 demonstrates the closeness of fitof the calculated and measured displacement, especially the maximumdisplacements, which is an important parameter.

FIGS. 3 and 4 show an exemplary mechanical layout of a hand-held modulecomprising a compression monitor 10, for example, implemented inaccordance with the schematic diagram shown in FIG. 1. The illustratedmodule 11 comprises a circular base 36 having an outer flange portion37. Mounted on base 36 is a circuit board 38. Circuit board 38 is fixedto base 36 by means of fasteners 40. A plurality of components aremounted directly on circuit board 38, including first and second gyros24 and 25, accelerometer 12, indicator 18, power source 20, andinterface 26.

The illustrated module is roughly 3 inches in diameter and 0.5 inches inheight. FIG. 3 shows first and second gyros 24 and 25 mounted at rightangles to each other on circuit board 38, which measure the angularvelocity around each of their respective longitudinal axes. Theillustrated accelerometer 12 is packaged in a TO-100 package (a 10 pincan), where the axis of sensitivity to acceleration (vertical) isperpendicular to the plane of circuit board 38. Accelerometer 12 isattached to a right angle support 43 which provides electricalconnections with circuit board 38, as well as a rigid mounting surface.

FIGS. 5-7 show various implementations of a hand-held device which maybe utilized in connection with the illustrated compression monitor 10disclosed herein.

FIG. 5 illustrates a rescuer 46 administering CPR to a recipient 47. Therescuer's hands are placed in contact with the recipient's chest at theproper location. A compression monitor 10 is attached to one of therescuer's wrists at the point which is proximate to the point at whichrescuer 46 is exerting force on the recipient's chest during CPR. Theillustrated monitor 10 comprises a mount coupled a housing portion ofthe monitor. In the illustrated embodiment, the mount comprises areleasable fixing mechanism, i.e., a band 48 for releasably fixinghousing portion 50 (containing the various components of monitor 10,such as those shown in FIG. 1) to the rescuer's 46 extremity (wrist).

In FIG. 6, a compression monitor 10 comprises a housing 50, and acompression force translating piece 52 positioned thereunder forfocusing the force exerted by rescuer 46 to a desired area downwardlyagainst the chest of the CPR recipient, in the direction facing therecipient's spine. The hand-held monitor 10 illustrated in FIG. 6 maycomprise a cable 44 for coupling monitored signals to a computing device(not shown) which is separate from the hand-held device. In thealternative, a processor may be integrally provided within housing 50,in which case cable 44 would not be necessary.

In FIG. 7, hand-held monitor 10 comprises a unitary disc-like housing50, upon which rescuer 46 places his or her hands. Each of the versionsof the compression monitor 10 shown in FIGS. 6 and 7 thus provides ontop of housing 50 a receiving portion for directly receiving adownwardly acting force from the hands of rescuer 46 proximate to apoint at which rescuer 46 is exerting force on the recipient's chestduring CPR. Depending upon whether housing 50 already contains amicroprocessor, an external cable 44 may be provided for coupling theelectrical components within housing 50 to, for example, an externalsignal monitoring system or a computer. ECG electrodes 54 are coupled torespective ECG signal lines 56 and an ECG monitor device (not shown).

A mechanism (e.g., a self-contained ECG display) may be provided withinthe illustrated compression monitor 10 for displaying and/or processingthe ECG signals; accordingly, alternatively, ECG signal lines 56 may becoupled to compression monitor 10.

In operation, the illustrated compression monitor 10 of either of theembodiments shown in FIGS. 5-7 will facilitate the effectiveadministration of CPR by producing a displacement-indicative signalindicative of the displacement of the recipient's chest toward therecipient's spine. Specifically, the audible indicator provided withindevice 10 is modulated to indicate when the proper chest displacement isachieved. That is, when a chest displacement in a desired range isachieved by rescuer 46, the audible indicator will output a modulatedsignal having a first pitch, while if the displacement is out of range,the frequency of the modulated signal will be at a second pitch. Theamplitude of the audible indication may be pulsed to coincide with thedesired frequency of chest compressions. Alternatively, the audibleindicator can provide, together, with appropriate signal processingcomponents, verbal indications to the rescuer 46, i.e., serving as voiceprompts to the rescuer. As another alternative, an audio transducer maybe provided which outputs a beeping sound to prompt the user to compressat the proper rate.

FIG. 8 shows a CPR recipient connected to various resuscitation-aidingapparatuses, including an automated constricting device 59 forautomatically administering CPR to the recipient. Automated constrictingdevice 59, more specifically, applies inwardly radial forces against therecipient's chest in order to cause a desired chest displacement in thedirection toward the recipient spine at a desired chest compressionfrequency.

Additional apparatuses connected to the recipient include a ventilatormask 58 coupled to an air tube 60, ECG electrodes and corresponding ECGsignal lines 56, defibrillation electrodes 62, and a CPR chestcompression monitor 10′ coupled to a cable 44, for carrying signalsgenerated thereby, including a detected acceleration signal.

The overall assembly facilities the resuscitation of a recipient 47 inan automated fashion. Such a set up can be particularly useful invarious situations, for example, including the case where the recipientis being carried in an ambulance vehicle. Resuscitation efforts could becontinued while the recipient is being transported, thus increasing thechance of survival by providing resuscitation efforts as soon aspossible while transporting the recipient to the hospital.

As illustrated, the recipient is hooked up to a ventilation apparatuscomprising a ventilator mask 58, which will allow respiration efforts tobe administered. The patient's ECG and associated heart rhythminformation can be monitored by ECG signal lines 56 coupled to an ECGmonitor device (not shown). CPR can be automatically administered byautomated constricting device 59. Timely defibrillation can beadministered with the use of defibrillation electrodes 62 coupled viadefibrillation lines 64 to a defibrillation device (not shown). Theautomated constricting device 59 can be controlled by signals producedby compression monitor 10′ so that the proper compression forces areapplied to the recipient's chest at the appropriate frequency.

In addition, the acceleration signal produced by compression monitor 10′can be retrieved via cable 44 and used to process the ECG signalobtained via ECG signal lines 56 concurrently with the administration ofCPR. More specifically, when CPR is administered, the ECG signal may beaffected and thus include a CPR-induced artifact. An ECG processor,which will be further described below, may be provided to process theECG signal so as to remove the CPR-induced artifact and render theresulting processed ECG signal meaningful and intelligible.

The automated constricting device 59 may comprise, for example, the CPRvest apparatus disclosed in the commonly-assigned co-pending patentapplication filed concurrently and on even date herewith in the name ofDr. Henry Halperin, or it may comprise an automated CPR system asdisclosed in U.S. Pat. No. 4,928,674 (Halperin et al). The content ofeach of these references is hereby expressly incorporated herein byreference in its entirety.

In the assembly shown in FIG. 8, an automated constriction controller(not shown) is provided together with the automated constricting devicefor applying CPR to the recipient 47 by applying a constricting force tothe chest of the recipient 47 under control of the automated controller.The automated controller receives the chest compression indicationsignals from compression monitor 10′, and, in accordance with the chestcompression indication signals, controls the force and frequency ofconstrictions applied to the CPR recipient's chest.

FIG. 9 is a flow chart illustrating a process for converting theacceleration and tilt signals produced by the compression monitor 10shown in FIG. 1 into a displacement-indicative signal, and forcalibrating the conversions. The illustrated process may be performedby, for example, microprocessor 28 as shown in the embodimentillustrated in FIG. 1.

In a first step S2, the acceleration signal is converted into a lineardisplacement x. Then, in step S4, the angular velocity signals output byeach of first and second gyros 24 and 25 are converted into respectiveangular displacements theta1 and theta2. In step S6, the displacement xis compensated for the tilting, thus producing a tilt-compensated lineardisplacement value x_(t) which is equal to x+ax(theta₁)+bx(theta₂).

During each chest compression cycle (usually 600-700 ms), the devicewill come to rest twice: at the zenith and nadir of the compression.These two time points may be easily identified since the verticalacceleration at these times will be 0, and there will be a change in thedirection of the velocity. Accordingly, at step S8, a determination ismade as to whether the device is at the zenith or nadir. If it is, thelinear displacement conversion is calibrated at step S10. If not, theprocess will return to step S2. In calibrating the linear displacementconversion, at step S10, measurements are made at the rest point tore-calibrate the system and eliminate the components v₀,x₀ from theequation (noted below) utilized to convert acceleration in to lineardisplacement x.

Algorithms are well known for converting an acceleration signal (from anaccelerometer) into linear displacement and for converting an angularvelocity signal (from gyros) into an angular displacement. In general,inertial navigation systems may determine position and orientation fromthe basic kinematic equations for transitional and rotational motion.The orientation of an object, given a sensed rotational rate, w, duringeach time step t, is given by:

θ=θ₀ +wt  (1)

where θ equals the orientation angle, t equals the time step and w isthe rotational rate output by a gyroscope.

Similarly, position is found with the transitional kinematic equation:

x=x ₀ +v ₀ t+(0.5)at ²,  (2)

where x equals position, v equals velocity and a equals acceleration,output by an accelerometer.

Motion and position may be estimated with equations, (1) and (2).Alternatively, motion and position may be estimated using a Kalmanfilter state estimation algorithm. Once the time-dependent motions andpositions of the system are estimated, a pattern recognition scheme suchas a neural network, hidden Markov model, or matched filter may beperformed with that motion data. The true vertical displacement x^(t)may be estimated as a combination of one translational and two angulardisplacements x, theta₁, and theta₂. It is expected that within theexpected angular deviation range of ±30 degrees from vertical a simpleequation (3) will work:

x _(t) =x+ax(theta₁)+bx(theta₂)  (3)

Coefficients a and b may be determined empirically using best linear fitmethods, or a more complex non-linear model, as appropriate.

In the event thermal drift is a factor, additional circuitry may beprovided as part of the compression monitor for thermal compensation.

While resuscitation is in progress, it is vital that health carepersonnel can be continuously aware of changes in the patient's ECG, inparticular the patient's heart rhythm. Incorrect assessment of the heartrhythm can lead to improper therapy. However, when CPR is administered,CPR-introduced artifacts will be present in the measured ECG signal thatmake interpretations difficult. FIGS. 10-16 provide various systemmodels, analysis waveform diagrams, and proposed ECG processingembodiments for addressing this problem.

As shown in FIG. 10, it can be assumed that the measured ECG signale_(m), for example, obtained on ECG signal lines 56, is equal to the sumof the true ECG signal e and the true CPR noise signal a (theCPR-induced artifact). A goal of the present invention is to provide asub-system, into which the measured ECG signal e_(m) is input, and fromwhich a processed measured ECG signal e_(m)′, absence the CPR-inducedartifact, is output.

As an initial approach toward eliminating the CPR-induced artifact, aband pass filter 66 as shown in FIG. 11 may be utilized. In thisapproach, the measured ECG em is viewed as the superposition of a trueECG e and CPR noise. Filter 66 selectively preserves as much of the ECGsignal as possible, while suppressing the artifact as well as possible.The problem with this approach is that it is difficult to separate thetrue ECG from the CPR-induced artifact since components of each of thosesignals coexist in the same portions of the frequency domain.

FIG. 12 shows several waveforms pertinent to the processing of aCPR-affected ECG signal. A first waveform a_(r) represents a measurablesignal which “represents” the CPR-induced artifact. That signal maycomprise a force, acceleration, distance, velocity, motion, or vestsignal, each of which represents some aspect of the CPR-inducedartifact. In the illustrated embodiment, the signal a_(r) comprises theacceleration signal produced by the accelerometer 12 of the device shownin FIG. 1.

The next waveform is the measure ECG signal e_(m), measured during CPR.The following waveform a_(p) is the predicted artifact. The lastwaveform e_(m)′, is the processed measured ECG signal, which has beenprocessed to remove the CPR-induced artifact. The processed measured ECGsignal e_(m)′ shown in FIG. 12 was produced using linear predictivefiltering as will be described below.

When a true ECG e and artifactual components a overlap in both time andfrequency domains, it is still possible to distinguish the two if aseparate signal that is correlated with the artifact is available. Thesystem that gives rise to the measured ECG signal e_(m) can be modeledas the sum of the true ECG e and an artifact waveform a. This model isshown in FIG. 13. The true CPR noise signal a is treated as the outputof a linear system {tilde over (H)} perturbed by a measurable inputa_(r).

The goal of linear predictive filtering, in accordance with theembodiment disclosed herein, is to identify the linear system H thattransforms the acceleration signal a_(r) into the waveform composed ofthe artifactural components, i.e., a, in the measured ECG e_(m). Oncethis system is identified, the artifactual component can be predicted,using linear perdictive filtering, by taking the output a_(p) of asimulated system H, using the acceleration signal a_(r) as the input.When this linearly predicted signal ap is substracted from the measuredECG e_(m), the resulting signal is the estimated true ECG, which isshown as the processed ECG signal e_(m)′ in the output of the systemshown in FIG. 14.

FIG. 15 shows a specific exemplary embodiment of the systemidentification process 70 shown in the embodiment of FIG. 14. The systemidentification block 70 comprises a correlated signal input a_(r) 86 anda non-correlated signal input e_(m) 88. Correlated signal input 86 isinput to a first FFT 76, while non-correlated signal input 88 is inputthrough a second FFT 78. The output of first FFT 76 is input to aautospectrum calculator 80 and to cross-spectrum calculator 82. Theoutput of second FFT 78 is input to cross-spectrum calculator 82.

The output of the first FFT 76 is the frequency domain representation ofthe measured signal a_(r) and the output of the second FFT 78 is thefrequency domain representation of the measured ECG signal e_(m).Autospectrum calculator 80 outputs Saa which is the input signal'sautospectrum, while cross-spectrum 82 outputs Sae which is thecross-spectrum between the observed input and output signals. These canbe computed using Fourier transform techniques, for example, asdisclosed by Jenkins et al. “Spectral Analysis and its Applications,”Holden-Day, Oakland, Calif. (1968), and R. D. Berger, “Analysis of theCardiovascular Control System Using Broad-Band Stimulation,” Ph.D.Thesis, MIT (1987), the content of each of which is hereby expresslyincorporated herein by reference in its entirety.

The input signals autospectrum Saa is then input into the denominatorinput of a complex divider 84, while the cross-spectrum Sae (between theobserved input and output signals) is input to the numerator input ofdivider 84. Divider 84 performs complex division on its input signals inorder to produce at its output 90 a complex representation of theestimated transfer function {tilde over (H)}. The transfer function{tilde over (H)} can be updated periodically from new short segments ofinput signals, which may include the acceleration signal output by theaccelerometer and the measured ECG signal. The processed ECG signale_(m) ′ output by the system shown in FIG. 14 is produced utilizing theoverlap and add technique.

Instead of system {tilde over (H)} being a linear system, a non-linearsystem may be estimated instead and used to subtract the CPR-inducedartifact from the measured ECG signal.

As an alternative to the systems shown in FIGS. 14 and 15, a recursiveleast squares (RLS) subsystem 90 may be provided as shown in FIG. 16.

In accordance with the recursive least squares method, each time a newdata sample is input to each of the inputs of the subsystem, therecursive model is modified on an ongoing basis. Techniques forutilizing the recursive least squares method to produce an RLS subsystem90 as shown in FIG. 16 are known in the art. For example, reference maybe made to L. Ljung et al., “Theory and Practice of RecursiveIdentification,” the MIT Press, Cambridge, Mass. (1986), the content ofwhich is hereby expressly incorporated by reference herein in itsentirety.

The following is an example program listing which may form the basis foremploying an RLS subsystem.

x: input(acceleration), y: measured output, z: predicted outputlinpred(x, y, z, npts, m, n) float *x, *y, *z; long npts; int m,n; /* m:MA order, n: AR order */ { double phi [MAXARMALEN], theta [MAXARMALEN],1[MAXARMALEN]; double p [MAXARMALEN][MAXARMALEN], alpha=1.0; doublearray1 [MAXARMALEN], array2 [MAXARMALEN], c; doublemat[MAXARMALEN][MAXARMALEN], mat2[MAXARMALEN] [MAXARMALEN], int i, j, k;for ( k=0; k<m+n; k++ ) { theta[k] = 0; for ( j= ; j<m+n; j++ ) { if (j—k ) p[k] [j] - LARGE; else p[k] [j] = 0; } } for(i-o; i<m+n, i++) z[i]= y[i]; for(i = m+n; i<npts; i++) { j=0; for ( k=1; k<=n; k++ ) { phi[j]= −y[i-k]; j++; } for ( k=1; k<=m; k++ ) { phi[j] = x[i-k]; j++; }mat_array_mult( p, phi, array1, m+n ); arrayt_array_mult( phi, array1,&c, m+n ); array_k_mult( array1, 1/alpha + c, 1, m+n ); arrayt_mat_mult(phi, p, array2, m+n ); array_arrayt_mult( 1, array2, mat1, m+n );mat_mat_subtract( p, mat1, mat2, m+n ); mat_copy( mat2, p, m+n );arrayt_array_mult( theta, phi, &c, m+n ); array_k_mult( 1, y[i]-c,array1, m+n ); array_array_add( theta, array1, array2, m+n );array_copy( array2, theta, m+n ); arrayt_array_mult( theta, phi, &c, m+n); z[i] = c; printf(“% 2f/n”, c); } } mat_array_mult(a, b, c, dim)double a[ ][MAXARMALEN], *b, *c; int dim; { int i, j; for(i = 0; i<dim;i++){ c[i] = 0 for(j-0; j<dim; j++) c[i] + = a[i][j] *b[j]; } }array_array_mult( a, b, c, dim ) double *a, *b, *c; int dim; [ int i: *c= 0; for( i= ; i<dim; i++ ) *c + = a[i] *b[i]; } array_mat_mult (a, b,c, dim) double *a, b[ ][MAXARMALEN], *c; int dim; { int i,j; for( i=0;i<dim; i++ ) { c[i] - 0; for ( j=0; j<dim; j++ ) c[i]+=a[j] *b[j][i]; }} array_arrayt_mult( a, b, c, dim ) double *a, *b, c[ ][MAXARMALEN]; intdim; { int i, j; for ( i=0; i<dim; i++ ) for ( j=0; j<dim; j++ )c[i][j] - a[i]*b[j]; } array_k_mult( a, b, c, dim ) double *a, b, *c;int dim; { int i; for( i=0; i<dim; i++ ) c[i] = a[i]*b; }mat_mat_subtract( a, b, c, dim ) double a[ ][MAXARMALEN], b[][MAXARMALEN], c[ ][MAXARMALEN]; int dim; { int i,j; for ( i=0; i<dim;i++ ) for ( j=0; j<dim; j++ ) c[i][j] = a[i][j]−b[i][j]; }array_array_add( a, b, c, dim ) double *a, *b, *c; int dim; { int i; for( i=0; i<dim; i++ ) c[i] = a[i]+b[i]; } mat_copy( a, b, dim ) double a[][MAXARMALEN], b[ ][MAXARMALEN]; int dim; { int i, j; for ( i=0; i<dim;i++ ) for (j =0; j<dim; j+1 ) b[i][i] = a[i][j]; } array_copy ( a, b,dim ) double *a, *b; int dim; { int i; for (i=0; i<dim; i++ ) b[i] =a[i];

While the invention has been described by way of example embodiments, itis understood that the words which have been used herein are words ofdescription, rather than words of limitation. Changes may be made,within the purview of the appended claims, without departing from thescope and spirit of the invention in its broader aspects. Although theinvention has been described herein with reference to particularstructures, materials, and embodiments, it is understood that theinvention is not limited to the particulars disclosed. Rather, theinvention extends to all appropriate equivalent structures, mechanisms,and uses.

What is claimed is:
 1. A system for facilitating the effectiveadministration of cardiopulmonary resuscitation (CPR), said systemcomprising: an accelerometer for producing an acceleration signalindicative of the displacement of a CPR recipient's chest toward therecipient's spine; a microprocessor being programmed to convert theacceleration signal into a distance value and compare the distance valueto a desired range; a signaling mechanism, operably connected to themicroprocessor, for indicating when the distance value is within thedesired range; a tilt compensator comprising a tilt sensor mechanism foroutputting a tilt compensation signal indicative of the extent of tiltof the accelerometer; and wherein the microprocessor is also programmedto adjust the distance value to account for the tilt compensationsignal.